Drop Sphinx 3, add Sphinx 5

[jarrodmillman]

See #65.

[codecov-commenter]

Codecov Report

Merging #72 (1c00dd5) into main (4cebac2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #72   +/-   ##
=======================================
  Coverage   88.90%   88.90%           
=======================================
  Files          10       10           
  Lines         910      910           
=======================================
  Hits          809      809           
  Misses        101      101           

Flag	Coverage Δ
pytests	88.90% <ø> (ø)

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[chrisjsewell]

superseded by #74